Lamp devices housing DC power source circuits and lighting circuits for lighting LEDs by DC power output from the DC power source circuits may be used in place of, for example, a straight-tube type or self-ballasted fluorescent lamp. The LEDs of the lamp device may each low power consumption and a long life. The lamp device detects voltage or current of the connected LEDs, performs feedback control on the lighting circuit and thus stably controls lighting of the LEDs. In this case, the DC power source circuit makes output voltage fixed or variable regardless of the voltage of the LEDs.